The present invention generally relates to a driving system and more particularly, to a three-dimensional driving arrangement for an objective lens in an optical disc player or the like.
Commonly, a reproducing apparatus for optical discs is so arranged that light radiated from a light source is converged or collected by an objective lens onto a row of pits on a recording disc, while a light spot is caused to accurately follow the row of pits, and thus, light reflected from the surface of the disc is received by an optical detector so as to read information through conversion of variation in light amount into electrical signals for reproduction.
By way of example, in a video disc player, it is required to effect control in a focusing direction so as to collect light on the pit row of the disc in a very small spot, and control in a tracking direction for causing the spot to accurately follow the pit row, and also, control in a tangential direction for removal of variation in time axis due to uneven rotation of the disc. For effecting controls in the three directions as described above, there may be conceived methods as follows.
(i) A method in which the focusing control is effected by driving an objective lens in a direction perpendicular to the disc surface, while two galvano-mirrors disposed in a light path between a light source and the objective lens are rotated to effect the tracking control and the tangential control,
(ii) A method in which an objective lens is two-dimensionally driven for effecting both of the focusing and tracking controls, while the tangential control is effected by one galvano-mirror, and
(iii) A method in which an objective lens is driven three-dimensionally.
However, the methods which employ the galvano-mirrors as in the above items (i) and (ii) have such disadvantages that, since the optical axis of the objective lens is inclined, the open aperture diameter of the lens must be increased, and that owing to the provision of the respectively independent control mechanisms, the apparatus tends to be large in size.
Accordingly, the method in the above item (iii) for driving the objective lens three-dimensionally will be further considered. In the first, there may be conceived a practice in which the objective lens is driven in the respective directions by independent driving mechanisms including magnetic circuits and coils, but this undesirably results in a large size of the resultant driving arrangement on the whole. Another practice which may be considered is a method in which three coils for the focusing control, tracking control and tangential control are provided in one magnetic gap for driving of the objective lens in three directions. In the above case, the driving arrangement may be reduced in size since only one magnetic circuit is sufficient for the purpose, but the way of winding of the coils is brought into question for effecting independent driving.
In connection with the above, there has conventionally been proposed, for example, in Japanese Patent Laid-Open Publication Tokkaisho No. 58-19743, one example of a three-dimensional driving arrangement in which three coils for the focusing control, tracking control and tangential control are disposed in one magnetic gap. In the actual construction, the known arrangement includes one focusing control coil, and four tracking and tangential control coils in total provided in one magnetic gap, and by feeding predetermined electric current to the focusing control coil, the objective lens support member is displaced in a Z direction. Meanwhile, with respect to the driving in the tracking direction, for example, if directions of currents for coils 3a to 3d are assumed to be as shown in FIG. 1, with magnitudes of the currents for the coils 3a and 3b and 3c and 3d being set to be equal to each other, forces fx are produced in the tracking direction. In this case, components fy of the forces in the tangential direction are offset to each other to render the resultant force to be zero. The driving in the tangential direction may also be effected based on the same principle as above. However, this practice has such disadvantages that the resultant force is not reduced to zero if there is any unbalancing of magnetic flux, or that independent driving can not be effected if it is required to drive in both directions simultaneously.